Powder slurry coatings provide the most desirable appearance with respect to smoothness of surface and gloss when prepared with powder coatings having the smallest average particle size attainable. Generally, in coatings known in the art, the average particle sizes range from 4 to 30 microns. While the art teaches that resin particles used for powder slurry coatings range in average particle size between 0.5 microns and 80 microns, it would be expected in the art that only 2 to 6% of the particles have a particle size of between 3 and 5 microns and 3 to 8% of the particles have a particle size of 10 microns. It is desirable that 100% of the particles are of a size less than or equal to 10 microns and at least 50% of the particles have a particle size of between 3 and 5 microns.
It is well known in the art to prepare a powder slurry by dispersing the pulverized powder coating into a mixture of water and surfactant. In some instances water soluble binder resins are used to disperse the powder. The mixture is then run through a mill to reduce the particles size of the coating. To this mixture water and dispersants are added. Generally, in the prior art, powder coatings produced according to this method have a particle size ranging between 5 and 105 microns at the start of the process. The larger particles are reduced by using a ball mill, sand mill, or Attritor (trademark of Attritor Company for a piece of grinding equipment providing fast, fine grinding), to reduce the particle size of the coating to achieve the best smoothness. It is difficult to achieve average particle sizes smaller than 18 microns with conventional grinding equipment including hammer and ACM mills, as these build up significant amounts of heat and require liquid nitrogen or other means to control heat build up.
Jet milling is known in the art for the production of toners and other very fine particle ink products. Generally jet milling is not utilized in the powder coating industry, as particles smaller than 20 microns tend to agglomerate in powder coatings. Fine powders are difficult to spray as they agglomerate into larger particles. Further, the resulting finish is very rough due to this agglomeration. When combining fine powders with water to make a powder slurry the powder is difficult to wet out in water and the resulting mixture tends to have powder floating on the surface with very little of the powder wetting out in the water. The little powder that does get into the water tends to have a "false body", which is a consistency of whipped cream or shaving cream. Also, the water and powder mixture can be formed only at low concentrations of solids. The resulting slurry obtained by this mixture is not sprayable.
The present invention provides a method for producing a powder slurry coating having an average particle size of less than 10 microns, wherein at least 50% of the particles are of a size of between 3 and 5 microns. This powder slurry coating does not require a grinding step once the coating is formed. Further, the method of the present invention solves the problems of particle agglomeration, low solids content and low viscosity. The slurry coatings do not use water soluble binders or solvents to form a film. The resultant coatings provide a very smooth film with a high degree of gloss.